Chemical nickel plating of magnesium and its alloys



United States Patent 3,121,644 CHEMICAL NICKEL PLATING 0F MAGNESIUM AND ITS ALLOYS Gregoire Gutzeit, Highland, and Warren G. Lee, Gary, Ind., assignors to General American Transportation Corporation, Chicago, 111., a corporation of New York No Drawing. Filed Dec. 15, 1961, Ser. No. 159,745 8 Claims. (Cl. 117-130) The present invention relates to chemical nickel plating of magnesium and its alloys, and more particularly to processes for effecting such plating with aqueous chemical nickel plating baths of the nickel cation-hypophosphite anion type.

Heretofore the chemical nickel plating of workpieces having surfaces formed essentially of magnesium and its alloys has been of limited utility as such coatings as have been produced thereon readily flake-otf since the coatings are not intimately bonded thereto. Also such coatings have been characterized by blisters indicating areas therebelow of no adhesion and complete separation from the adjacent metal surfaces of the workpieces.

Perhaps the principal reason for this lack of adhesion between this metal surface and the coating is that the metal surface is highly susceptible to oxidation and filmsover prior to or upon contact with the aqueous chemical nickel plating bath; whereby there is a film at the interface between the magnesium metal surface and the coating of nickel-phosphorus alloy that is inherently produced by a plating bath of this type; which film at the interface prevents proper initiation of the plating and intimate bonding between the magnesium alloy metal surface and the coating, with the result that the coating is characterized by blisters and other defects evidencing lack of continuity and uniformity of adhesion thereof with respect to the metal surface of the workpiece.

Furthermore, the production of desirable coatings upon such magnesium alloy surfaces has been aggravated by the general utilization of a chemical nickel plating bath having a pH in the acid range, as magnesium alloys are inherently sensitive to attack by acid solution even at room temperature; which attack is greatly increased at the normal elevated temperature of operation of the chemical nickel plating bath. Of course, the chemical nickel plating bath in the acid range mentioned has been employed heretofore, since such a plating bath in the acid range is quite stable; whereas such a chemical nickel plating bath in the alkaline range is notoriously unstable. The term stable as employed above means that the chemical nickel plating bath is not subject to spontaneous decomposition by the rapid and random formation thcrethrough of black precipitate.

Accordingly, it is a general object of the invention to provide a process of chemical nickel plating of a workpiece having a metal surface formed essentially of 'magnesium, wherein the coating produced is intimately bonded to the metal surface and exhibits great adhesion thereto.

Another object of the invention is to provide a process of coating the magnesium alloy surface of a workpiece with a chemical nickel plating bath of the nickel cationhypophosphite anion type so that the coating thus produced is intimately bonded thereto and devoid of blisters or other defects characteristic of lack of adhesion.

Another object of the invention is to provide a process of chemical nickel plating upon a previously prepared magnesium alloy surface of a workpiece that is productive of a continuous smooth coating of nickel-phosphorus alloy that is devoid of blisters and other defects characteristic of lack of adhesion.

A further object of the invention is to provide a process of the character noted that employs a chemical nickel plating bath of the nickel cation-hypophosphite anion type of improved composition which has a pH in the alkaline range and that is entirely stable notwithstanding this circumstance.

Another object of the invention is to provide a process of the character noted that utilizes an improved lowtemperature heat-treating step following the chemical nickel plating of the coating upon the metal surface of the workpiece, whereby the coating is intimately bonded to the underlying metal surface of the workpiece.

A further object of the invention is to provide a chemical nickel plating bath of the nickel cation-hypophosphite anion type and of improved composition which has a pH in the alkaline range and that is entirely stable notwithstanding this circumstance.

A still further object of the invention is to provide an improved process of producing, as an article of manufacture, a workpiece having a metal outer surface formed essentially of magnesium alloy and carrying a smooth and continuous coating intimately bonded thereto and essentially comprising a nickel-phosphorus alloy.

Further features of the invention pertain to the particular arrangement of the steps of the process and of the elements of the chemical nickel plating bath, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood as the following specification proceeds.

In accordance with the present invention, there is provided a workpiece or article of manufacture having an outer surface formed essentially of magnesium or an alloy thereof; and ordinarily the workpiece is first ma chined, or otherwise finished, and thereafter subjected to the process so as to provide on the metal surface thereof a continuous uniform and smooth coating intimately bonded thereto and formed of a nickel-phosphorus ally that is inherently produced by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type. Specifically, after the workpiece 'is machined, or otherwise finished, it is first subjected to certain pretreatment steps, then it is subjected to chemical deposition from a first plating bath of the type noted having a pH in the alkaline range, then it is subjected to chemical deposition from a second plating bath of the type noted having a pH in the acid range, and ultimately it is subjected to a heat-treatment step in order to render the composite coating product in the two chemical nickel plating steps mentioned intimately bonded thereto and of finished character.

Specifically, the present process has been applied in coating the typical magnesium alloys: AZ-3l, ZK-60A, AZ-9l, AZ-6l, AZ-92 and AZ-80. These typical magnesium alloys have the following compositions by weight.

AZ-31: Percent A1 3 Zn 1 Mg Balance ZK-60A:

Zn 5.5 Zr 0.45 Mn 0.06 Mg Balance Al 9.5 Zn 0.4 Mg Balance AZ-611 Percent Al 6 Zn Mg Balance Al 9 Zn 2 Mn 0. Mg Balance Al Zn Mg Balance The alloy AZ-91 is also identified as AZ91A and comprises a casting alloy; the alloy AZ-61 corresponds to Dow alloy K-l and comprises an extrusion alloy; and the alloy AZ-8l corresponds to Dow alloy C l and comprises a forging alloy.

In the pretreatment, the workpiece is subjected to the following steps in the order named:

(1) Subject to standard vapor degreasing.

(2) Subject to soaking in an aqueous solution of a suitable cleaner, such as Enthone S160 (about 60 gms. per liter) at an elevated temperature of about 75 C. for a time interval of about 20 mins.

(3) Rinse with water at ambient temperature for a time interval of about 1 min.

(4) Pickle in pickling bath No. 1, described below, at room temperature for a time interval of about 3 mins.

(5) Rinse with water at ambient temperature for a time interval of about 1 min.

(6) Pickle in pickling bath No. 2, described below, at room temperature for a time interval of about 4 mins.

(7) Rinse with water at ambient temperature for a time interval of about 1 min.

(8) Subject to soaking in treatment bath No. 3, described below, at a temperature of about 80 C. for a-time interval of about 5 mins.

(9) Rinse with water at ambient temperature for a time interval of about 2 to 3 mins.

PICKLING BATH NO. 1

This pickling bath employed in the pretreatment essentially comprises an aqueous solution of chromic acid, nitric acid and hydrofluoric acid and has the general composition:

M.p.l. CrO About 2.8 HNO About 015 HP About 0.04

This bath may be conveniently formulated by dissolving in water about 280 g./l. of CrO and by adding thereto 10 ml./l. of 42 B. (technical) HNO and by adding thereto 1 ml./l. of an aqueous solution of 70% HF.

PICKLING BATH NO. 2

TREATMENT BATH NO. 3

This treatment bath employed in the pretreatment essentially comprises an aqueous solution of sodium hydroxide and has the general composition: NaOH-about 0.1 m.p.l.

This bath may be conveniently formulated as a 10% sodium hydroxide aqueous solution.

Reconsidering the pretreatment, it is pointed out that magnesium, as well as its alloys, is an extremely reactive metal, which is substantially attacked by water at temperatures as low as 70 C., and that the rate of attack is substantially increased at elevated temperatures. Therefore,

4 to achieve cleaning and pickling, but also to achieve the formation of a protective film upon the exterior surface of the magnesium workpiece which will slow-down or pre vent this corrosive attack in the first chemical nickel plating bath. In other words, the magnesium surface must be passivated, as well as cleaned and pickled.

In the pretreatment, the pickling bath No. l dissolves all the magnesium oxides from the magnesium surface of the workpiece. Moreover, there is a chemical polishing action, due to the presence of chromic acid (an oxidizing agent), which, in conjunction with the nitric and hydrofiuoric acids, attacks the convex irregularities of the surface at a higher rate.

In the pretreatment, the pickling bath No. 2 first attacks the clean metal surface obtained by the pickling bath N0. 1, as shown by hydrogen evolution. Hydrogen evolution then subsides, and an insoluble film of magnesium fluoride is formed upon the clean magnesium surface of the workpiece.

In the pretreatment, the treatment bath No. 3 effects an alternation of the film upon the magnesium surface that was deposited thereupon by the pickling bath No. 2; and more particularly, the treatment hath No. 3 first neutralizes any acid that may be present on the surface of the workpiece, and then alters the magnesium fluoride fihn upon the magnesium surface of the workpiece and forms a final protective film of basic magnesium fluoridemagnesium hydroxide thereon. It has been found that this film upon the magnesium surface of the workpiece initiates plating very rapidly in the first chemical nickel plating bath previously mentioned and is also highly useful in obtaining adhesion of the coating and lack of blistering thereof upon the workpiece.

After the workpiece has been subjected to pretreatment, as described above, it is transferred to a first chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH in the alkaline range and immersed therein throughout a first time interval sufficiently long to produce an initial nickel coating having a thickness of at least about 0.1 mil upon the exterior surface thereof; which first plating bath is normally maintained at a relatively high temperature in the general range 92 C. to 98 C. The initial plating or coating that is inherently produced by this first plating bath essentially comprises about 92% to 97% nickel and about 3% .to 8% phosphorus by weight; which plating proceeds at a rate of at least 1.0 mil/hour.

This first plating bath essentially comprises an aqueous solution of nickel cations, hypophosphite anions, a first chelating and exalting agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof, a second chelating agent selected from the class consisting .of short chain aliphatic hydroxycarboxylic acids and salts thereof, ammonium cations, fluoride anions, sufiicient hydroxide anions (OH") to produce a pH in the approximate range 7.0 to 9.0, a buffering system comprising alkali metal cations and boron oxide anions, and AsO anions.

This first plating bath contains Ni++ and H Po,, the absolute concentration of H2PO2 being in the range 0.15 to 1.20 moles per liter, and the ratio between Ni and H PO being in the rang 0.25 to 1.60.

This first plating bath has the general composition:

Ni++ 0.07 to 0.09 m.p.l. H PO 0.21 to 0.27 m.p.l. Aminocarboxylic acid radical 0.08 to 0.12 m.p.l. Hydroxycarboxylic acid radical 0.06 to 0.10 m.p.l. NH 0.05 to 0.20 m.p.l. F- 0.10 to 0.40 m.p.l. NaOH to adjust pH 7.0 to 9.0.

Boron oxide anions equivalent to an addition of alkali metal tetraborate 0.05 to 0.20 m.p.l.

the fundamental purpose of the pretreatment is not only (AsO Stabilizing content.

The preferred composition of this first plating bath is as follows:

Suitable examples of this first plating bath are as follows:

Example N0. 1

NiSO .6H O m.p.l. 0.08 NHH POgJ'I Q m.p.l. 0.23 Aminoacetic acid m.p.l. 0.10 Citric acid m.p.l. 0.08 NH HF, m.p.l. 0.10 NaOH to adjust pH 8.0 to 9.0 Na B O .10H O m.p.l. 0.10 AS205 to produce (ASO -p.p.m. 30 to 150 Example N0. 2 Nickel Hypophosphite ..m.p.l. 0.08 Hypophosphorous acid m.p.l. 0.07 Aminoacetic acid m.p.l. 0.10 Citric acid m.p.l. 0.08 Ammonium fluoride m.p.l.. 0.10 Sodium fluoride rn.p.l. 0.10 NaOH to adjust pH 8.0 to 9.0 NagB407-10H O .m.p.l. As O to produce (AsO p.p.m... 30 to 150 Example N0. 3 Nickel sulfate m.p.l. 0.08 Sodium hypophosphite m.p.'l.. 0.23 Aminosuccinic acid m.p.l... 0.10 Hydroxyacctic acid m.p.1. 0.08 NH HF m.p.l. 0.10 NaOH to adjust pH 8.0 to 9.0 Na B O- 10H O m.p.l. 0.10 As O to produce (AsO p.p.m. 30 to 150 Example No.4

Nickel sulfate -Q, m.p.-l 0.08 Sodium hypophosphite -m.p.l 0.23 Aminoacetic acid m.p.l.. 0.10 Citric acid m.p.l 0.08 Hydrofluoric acid m.p.'1 0.20 NaOH to adjust pH 8.0 to 9.0 Orthoboric acid m.p.l 0.40 A5 to produce (AsO p.p.m 30 to 150 In this first plating bath, the Ni++ may be derived from nickel hypophosphite, nickel acetate, nickel sulfate, nickel carbonate, etc.; and the H POr may be derived from nickel hypophosphite, hypophosphorous acid, alkali metal hypophosphite, etc. The NH may be derived from ammonium hydroxide, ammonium fluoride, ammonium sulfate, ammonium bifluoride, etc.; and the F- may be derived from hydrofluoric acid, ammonium fluoride, ammonium bifluor-ide, alkali metal fluoride, etc. The short chain aliphatic aminocarboxylic acid radical may be derived from either the corresponding acid or the alkali metal salt thereof; this class of acids includes aminoacetic acid, alpha-aminopropionic acid, beta-aminopropionic acid, alpha-aminobutyric acid, aminosuccinic acid, etc. The short chain aliphatic hydroxycarboxylic acid radical may be derived from either the corresponding acid or the alkali metal salt thereof; this class of acids includes hydroxyacetic acid, monohydroxysuccinic acid, dihydroxysuccinic acid, gluconic acid, citric acid, hydroxymalonic acid, trihydroxyglutaric acid, alpha-hydroxypropionic acid, beta-hydroxypropionic acid, l-beta-hydroxybutyric acid, etc. The hydroxyl ions may be derived from ammonium hydroxide, or a suitable alkali metal hydroxide, sodium hydroxide being preferred; and any H+ adjustment of the pH is preferably derived from H The AsO,,-- anions may be conveniently derived by dissolving As 0 into the bath, the AsO anion being stable in the alkaline range of the bath. The boron oxide anions may be derived from H BO H B O Na BO or Na B O or any desired combination thereof.

In this first plating bath, the aminocarboxylic acid radical constitutes a first chelating agent, as well as an exaltant increasing the rate of deposition; and. the hydroxycarboxylic acid radical constitutes a second chelating agent; which two chelating agents named are present in amounts sufficient to form mixed chelates of all of the nickel ions in the bath. Moreover, the NH constitutes a complexing agent, thereby to insure that all of the nickel ions are tied-up or complexed either by way of the mixed chelates mentioned or by way of the ammoniacal complexes mentioned. In the bath, the FT is employed fundamentally for the purpose of preventing blistering of the first coating as it is applied to the magnesium metal surface of the workpiece.

This first plating bath includes the fundamental buffering system comprising alkali metal cations (Na+) and boron oxide anions. Specifically, the plating bath inherently contains a substantial concentration of Na+ cations by virtue of the utilization of the sodium salts of the various acids involved in composing the same and by the utilization of NaOH in establishing the pH thereof in the alkaline range. However, in the event of insufflcient Na+ cations in the plating bath, the concentration thereof may be readily adjusted utilizing a suitable sodium salt such, for example, as sodium sulfate.

Also this first plating bath includes two auxiliary buffering systems. The first auxiliary buffering system comprises alkali metal cations (Na+) and the aminocarboxylic acid radical; and the second auxiliary buffering system comprises alkali metal cations (Na+) and the hydroxycarboxylic acid radical.

Accordingly, it will be appreciated that this first plating bath is highly buffered so as accurately to maintain the desired pH thereof within the range 8.0 to 9.0 as previously noted.

Reverting to the fundamental buffering system, it is pointed out that this first plating bath comprises a boron oxide-sodium oxide-water system; whereby the source of the boron oxides present therein is not only immaterial, but the ultimate composition of the ionic species of the boron oxides in the plating bath is always the same, irrespective of the source of boxon oxide employed, under the same controlling equilibria conditions of concentration, temperature and pH. These propositions will best be understood from an examination of the fundamental reactions involved in this system as represented by the following equations:

Considering more particularly this system, it is noted that a 0.6 M aqueous solution of H BO essentially comprises:

M Of HgB O-1 8.0)(10 M of B 0, 4.0X10- M of H2BO3 Balance principally H B0 7 On the other hand, a 0.1 M aqueous solution of H BO essentially comprises:

6.0 10- M of H B O 3.0)(10 M of B 8.0 M of H BO Balance principally H BO Thus with increasing dilution of the H BO the equilibria shifts from B O-r toward H BO Further considering this system, it is noted that a dilute aqueous solution prepared by a Na B4O addition essentially comprises:

On the other hand, an increasing dilution of Na B O causes this aqueous solution essentially to comprise:

In an aqueous solution prepared by an addition of 0.1 m.p.l. of Na B O .10H O, the resulting pH is 9.2, and the dominant constituents are Na B O and H 80 Now when the pH of this aqueous solution is adjusted to 6.0 with H 80 the dominant constituents are H BO and B0 From Equation No. 6 above, it will be observed that a high pH of the system favors the formation of H BO and from Equation No. 7 above, it will be observed that a low pH of the system favors the formationof B O Moreover, the Equations Nos. 6 and 7 above are in no way dependent upon the source or origin of the H BO and the B O whereby the equilibrium is dependent upon pH, but is not in any way dependent upon the source or origin of the boron oxides of the Equations Nos. 1 to 4, inclusive, above.

Furthermore, since in every case some B O is present, the high concentration of Na+ in the plating bath always causes the production therein of some undissociated Na B O Since this first plating bath is always employed in a plating operation in the general temperature range 92 C. to 98 C, temperature considerations controlling equilibria therein of the ionic species of the boron oxides require no particular discussion; and likewise, the concentration considerations controlling equilibria therein of the ionic species of the boron oxides require no particular discussion, since the concentrations are employed within the fixed range of equivalents among these sources of boron oxide ions. More particularly, the plating bath contains boron oxide anions that are equivalent to an addition of Na B O in the range 0.05 to 0.20 m.p.l. This content of boron oxide is also equivalent to an addition of H 80 in the range of 0.20 to 0.80 m.p.l. While any one of these sources of boron oxide may be employed, as previously explained, the utilization of ordinary borax is recommended, because of factors pertaining to simplicity and economy.

In this first plating bath, the AsOpanions are employed for the purpose of achieving stability thereof so as to prevent spontaneous decomposition of the first plating bath by the rapid and random formation therethrough of black precipitate. The stabilizing content of ASO4 anions is ordinarily within the range to 150 parts per 1,000,000 parts of the bath by weight.

In the utilization of this first plating bath, nickel cations and hypophosphite anions are depleted; whereby either continuously or periodically the bath is regenerated in use by the addition of these ingredients noted in order to maintain the ranges thereof set forth.

After the workpiece has been given an initial coating having a thickness of at least about 0.1 mil in the first chemical nickel plating bath described above, it is transferred to a second chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH in the acid range and immersed therein throughout a second time interval sufficiently long to produce a final composite nickel coating having the desired thickness upon the exterior surface thereof; which second plating bath is normally maintained at a relatively high temperature in the general range 93 C. to 98 C. The plating or coating that is inherently produced by this second plating bath essentially comprises about 88% to 94% nickel and about 6% to 12% phosphorus by weight; which plating proceeds at a rate of about 0.9 mil/hour.

This second chemical nickel plating bath is preferably of the composition of that disclosed in US. Patent No. 2,822,294, granted on February 4, 1958, to Gregoire Gutzeit, Paul Talmey and Warren G. Lee; which plating bath is essentially of the nickel cation-hypophosphite anion type also containing lactic anion and propionic anion and having a pH in the acid range 4.0 to 6.0. Specifically, this plating bath essentially comprises nickel cations, hypophosphite anions within the range 0.15 to 1.20 moles/ liter, a ratio between nickel cations and hypophosphite anions Within the range 0.25 to 1.60, lactic anions within the range 0.25 to 0.60 mole/liter, and propionic anions within the range 0.025 to 0.060 mole/ liter. A typical example of this second chemical nickel plating bath has the following composition:

NiSO .6H 0 0.08 m.p.l. NaH PO 0.23 m.p.l. Lactic anion 0.30 m.p.l. Propionic anion 0.03 m.p.l. Pb++ About 1 p.p.m. pH 4.6.

In the utilization of this second chemical nickel plating bath, nickel cations and hypophosphite anions are depleted; whereby either continuously or periodically the bath is regenerated in use by the addition of the ingredients noted in order to maintain the ranges thereof set forth.

In carrying out the present process, after the required thickness of the composite plating has been chemically deposited upon the workpiece in the above described second chemical nickel plating bath, it is removed therefrom, rinsed with water and then subjected to heattreatment. More particularly, the workpiece carrying the composite plating or coating is heated to a temperature of about 400 F. for a time interval of about 1 hour; which heat-treatment effects intimate bonding between the chemically deposited composite plating and the magnesium metal surface of the workpiece so that an adherent, smooth, continuous and uniform coating is provided thereon.

In carrying out the present process, it is very important that the initial coating be deposited upon the magnesium surface of the workpiece in the first chemical nickel plating bath having a pH in the alkaline range, and that this step be continued throughout a suitable time interval in order to obtain a thickness of at least about 0.1 mil of the initial coating mentioned, thereby to prevent attack of the magnesium metal surface of the workpiece. Thereafter, it is quite satisfactory to transfer the workpiece to the second chemical nickel plating bath having the pH in the acid range in order to effect an increase in the thickness of the initial coating into the required range that is ordinarily from about 1 to 3 mils, since the initial coating mentioned prevents attack upon the magnesium metal surface of the workpiece. This procedure is recommended since the second chemical nickel plating bath is, of course, very simple, exceedingly easy to control and altogether economical with respect to the first chemical nickel plating bath. Of course the transfer of the workpiece from the first chemical nickel plating bath to the second chemical nickel plating bath is not essential but is recommended because of the foregoing factors.

Again reconsidering the first chemical nickel plating bath, it is pointed out that while the same may be stabilized utilizing Pb++ cations as the stabilizing agent in the general range 1 to p.p.m., there is a tendency for the Pb++ cations to become depleted by the precipitation of Pb(OH) On the other hand, the utilization of ASO4 anions as the stabilizing agent in the range 30 to 150 ppm. is very advantageous, since there is no tendency for the AsOp anions to become depleted by precipitation.

In carrying out the present process, it is emphasized that the heat-treatment step is very important as it not only integrates the composite coating upon the magnesium meal surface of the workpiece, but it also materially enhances the adhesion so that such a workpiece having a coating thickness from 1 to 3 mils has an average salt spray life in the approximate range 120 to 250 hours, depending upon the thickness of the final coating.

When the present process is carried out in the manner described, smooth, bright coatings are obtained that are entirely devoid of blisters or other defects characteristic of lack of adhesion, which coatings are intimately bonded to the workpieces.

In view of the foregoing, it is apparent that there has been provided an improved process of producing coatings of nickel-phosphorus alloy upon workpieces having surfaces formed of magnesium and its alloys, which coatings are smooth, bright, uniform, continuous and intimately bonded to the workpieces. Moreover, the process involves improved pretreatment steps, plating steps and heat-treatment steps that are correlated and coordinated to obtain the results aforesaid.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An aqueous chemical nickel plating bath comprising nickel ions, hypophosite ions in the range 0.15 to 1.20 moles per liter, the ratio between nickel ions and hypophosphite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof and in the range 0.08 to 0.12 mole per liter, a second chelating agent selected from the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof and in the range 0.06 to 0.10 mole per liter, ammonium ions in the range 0.05 to 0.20 mole per liter, fluoride ions in the range 0.10 to 0.40 mole per liter, sufiicient hydroxyl ions to produce a pH in the approximate range 7.0 to 9.0, and a buffering system comprising alkali metal cations and boron oxide anions and equivalent to an addition of alkali metal tetraborate in the range 0.05 to 0.20 mole per liter.

2. An aqueous chemical nickel plating bath comprising nickel ions, hypophosphite ions in the range 0.15 to 1.20 moles per liter, the ratio between nickel ions and hypophosphite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of arni-noacetic acid and salts thereof and in the range 0.08 to 0.12 mole per liter, a second chelating agent selected from the class consisting of citric acid and salts thereof and in the range 0.06 to 0.10 mole per liter, ammonium ions in the range 0.05 to 0.20 mole per liter, fluoride ions in the range 0.10 to 0.40 mole per liter, sufiicient hydroxyl ions to produce a pH in the approximate range 7.0 to 9.0, and a buffering system comprising alkali metal cations and boron oxide anions and equivalent to an addition of alkali metal tetraborate in the range 0.05 to 0.20 mole per liter.

3. An aqueous chemical nickel plating bath comprising nickel ions, hypophosphite ions in the range 0.15 to 1.20 moles per liter, the ratio between nickel ions and hyposphophite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof and in the range 0.08 to 0.12 mole per liter, a second chelating agent selected trom the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof and in the range 0.06 to 0.10 mole per liter, ammonium bifluoride in the range 0.05 to 0.20 mole per liter, sufficient hydroxyl ions to produce a pH in the approximate range 7.0 to 9.0, and a. buffering system comprising alkali metal cations and boron oxide and equivalent to an addition of alkali metal tetraborate in the range 0.05 to 0.20 mole per liter.

4. The process of plating with nickel the metal surface of a workpiece, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning and pickling said metal suruface, and then contacting said metal surface with a hot aqueous chemical nickel plating bath during a time interval sufiioiently long to produce a nickel coating of desired thickness upon said metal surface, wherein said aqueous chemical nickel plating bath comprises the composition set forth in claim 1.

5. The process of plating with nickel the metal surface of a workpiece, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning and pickling said metal surface, then contacting said metal surface with a hot aqueous chemical nickel plating bath during a time interval sufiiciently long to produce a nickel coating of desired thickness upon said metal surface, wherein said aqueous chemical nickel plating bath comprises the composition set forth in claim 1, and then heat-treating said workpiece at a temperature of about 400 F. for a time interval of about one hour.

6. The method of plating with nickel the metal surface of a workpiece, wherein said metal surface is formed essentially of magnesium; said method comprising cleaning and pickling said metal surface, then immersing said workpiece in a first aqueous chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH on the alkaline side during a first time interval sufficiently long to produce an initial nickel coating having a thickness of at least about 0.1 mil upon said metal surface, and then immersing said workpiece: in a second aqueous chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH on the acid side during a second time interval sufficiently long to produce a final bright nickel coating of desired thickness upon said metal surface; wherein each of said baths comprises nickel ions, hypophtosphite ions in the range 0.15

to 1.20 moles/liter, and a ratio between nickel ions and hypophosphit'e ions in the range 0.25 to 1.60.

7. The method set forth in claim 6, wherein said first bath has a pH in the approximate range 8.0 to 9.0, and said second bath has a pH in the approximate range 4.0 to 6.0. i

8. The method of plating with nickel the metal surface of a workpiece, wherein said metal surface is formed essentially of magnesium; said method comprising cleaning and pickling said metal surface, then immersing said workpiece in a first aqueous chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH on the alkaline side during a first time interval sufficiently long to produce an initial nickel coating having a thickness of at least about 0.1 mil upon said metal surface, then immersing said workpiece in a second aqueous chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH on the acid side during a second time interval sufficiently long to produce a final nickel coating of desired thickness upon said metal surface, and then heat-treating said workpiece at a temperature of about 400 F. for a time interval of about one hour; wherein each of said baths comprises nickel ions, ltypophosphite ions in the range 0.15 to 1.20

11 12 moles/liter, and a ratio between nickel ions and hypo- 2,762,723 Talmey at al Sept. 11, 1956 phosphite ions in the range 0.25 to 1.60. 2,766,138 Talmey Oct. 9, 1956 2,884,344 Ramirez Apr. 28, 1959 References Cited in the file of this patent 2,916,401 l et 1 3, 1959 UNITED STATES PATENTS 5 2,935,425 Gutzeit et a1. May 3, 1960 2,983,634 Bu'dininkas May 9, 1961 2,694,017 Reschan e1 :11 Nov. 9, 1954 2,994,369 0min Aug 1, 1961 

8. THE METHOD OF PLATING WITH NICKEL THE METAL SURFACE OF A WORKPIECE, WHEREIN SAID METAL SURFACE IS FORMED ESSENTIALLY OF MAGNESIUM; SAID METHOD COMPRISING CLEANING AND PICKLING SAID METAL SURFACE, THEN IMMERSING SAID WORKPIECE IN A FIRST AQUEOUS CHEMICAL NICKEL PLATING BATH OF THE NICKEL CATION-HYPOPHOSPHITE ANION TYPE HAVING A PH ON THE ALKALINE SIDE DURING A FIRST TIME INTERVAL SUFFICIENTLY LONG TO PRODUCE AN INITIAL NICKEL COATING HAVING A THICKNESS OF AT LEAST ABOUT 0.1 MIL UPON SAID METAL SURFACE, THEN IMMERSING SAID WORKPIECE IN A SECOND AQUEOUS CHEMICAL NICKEL PLATING BATH OF THE NICKEL CATION-HYPOPOSPHITE ANION TYPE HAVING A PH ON THE ACID SIDE DURING A SECOND TIME INTERVAL SUFFICIENTLY LONG TO PRODUCE A FINAL NICKEL COATING OF DESIRED THICKNESS UPON SAID METAL SURFACE, AND THEN HEAT-TREATING SAID WORKPIECE AT A TEMPERATURE OF ABOUT 400*F. FOR A TIME INTERVAL OF ABOUT ONE HOUR; WHEREIN EACH OF SAID BATHS COMPRISES NICKEL IONS, HYPOPHOSPHITE IONS IN THE RANGE OF 0.15 TO 1.20 MOLES/LITER, AND A RATIO BETWEEN NICKEL IONS AND HYPOPHOSPHITE IONS IN THE RANGE OF 0.25 TO 1.60. 